The main inspiration for the FF was Jack Kirby's earlier series Challengers of the Unknown for DC. Both feature teams of four heroes whose abilities reflect the four classical elements, but in the former case they were non-powered men with different specialties (a miner, a pilot, a firefighter, and a diver, or something like that) while in the latter they were given superpowers reflecting the elements (changeable Reed is water, invisible Sue is air, and the other two are obvious).